Reference to respective STA endpoints

To implement the functionality requested in hub-terra/sms/service-desk#71, it is necessary to reference the respective STA endpoints in TSMDL. Due to different base URLs and the lack of standardized URL schemes for Datasources, TSMDL needs to provide information on how to access the STA endpoint of a Datasource.

However, references to Things and Datastreams can be generated directly from the Datasource, as they stick to standardized STA and OData URL conventions. Therefore, including an explicit reference to these would not be necessary.